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Real Action Paintball Mortar

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Real Action Paintball Mortar

Unread postAuthor: Hubb » Thu Aug 28, 2008 1:58 pm

Okay, guys. I've been working on a a real action mortar. Here is what I've come up with so far:

I use a 2" SDR pipe as the barrel. This allows my ammo made for 2" Sch40 pipe to slide down without obstruction. The valve is triggered by an electric sprinkler valve. The switch is a homemade switch, and is the key to making this work like a real mortar.

The switch is made by drilling a hole on one side of the pipe, and a slit (the hole's diameter) on the opposite side, about 1/4" long. I ran a piece of coathanger wire through the hole / slit and bent it to keep it from falling out. About another 1/4" from the slit, on the back side, I drilled a hole and secured a small piece of metal with a bolt. The bolt was connected to one wire from the solenoid. The end of the coathanger that's closest to the hole is secured to the other wire from the solenoid.

I've included a diagram to better help explain.
Image

The chamber is made from 1.5" Sch40 pipe. The whole thing is about 2.5' long. Ratio stands at about .5:1.

My air source is liquid propane. I chose this because its operating pressure is between 90 and 110 psi, which means I don't need a regulator on a constant air source.

Here is how it works:
The propane valve is turned on, filling the chamber to pressure and seating the valve. A round is dropped into the barrel and slides down. The round pushes on the rod, which completes an electrical connection, firing the solenoid. The round is fired, the connection is broken, the valve reseats, and the chamber repressurizes and awaits the next round.

Right now, I've got some bugs to work out. You see, I'm loosing power due to the solenoid not staying open long enough. Not only that, but the refill time on the chamber is about 10 seconds. This is no big deal, but I'd like it to be shorter without compromising the chamber volume. If anyone wants to provide suggestions for these, feel free to do so.
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Unread postAuthor: BC Pneumatics » Thu Aug 28, 2008 2:14 pm

2" SDR and 2" Sch 40 are the same OD, did you mean 2.5" SDR 21 perhaps? Aside from making sure that every component you use will stand up to propane (some rubbers will deteriorate, meaning you have you check the materials that all your seals are made from), you are going to have to deal with the venting of large quantities of propane.

I have used propane as a pneumatic source before, but never got around to the necessary flame testing, because the launch stunk too much to be of much use. Dumping large amounts of propane has some problems.

Also, to stay inside your 90-110psi range without regulation, you will only be able to fire your launcher with a tank temperature between 58*F and 70*F. Rapid fire will cool the tank, and change the internal pressure (which means altering the firing distance). This is particularly bad for a mortar, as the first shot is many times nothing more than a standard by which to adjust for the second shot.

I have spent some considerable time looking into LP as a gas for Airsoft/PB use, and after some cash and a lot of tinkering, I concluded that there is a reason the hobby's multi-million dollar leaders have never picked it up for use as a non-combusting propellant.

The novelty of it makes the project fun, especially if you tape a bic to the end, but if you are looking for something practical you may want to consider your other options.

Just my two cents. Hopefully it is constructive, and not demeaning. :)
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Unread postAuthor: kablooie » Thu Aug 28, 2008 2:47 pm

This sounds like it could be cool, although kind of unnecessary. Props for doing something different, I look forward to a video. Another idea that would be powerful would be to simply have a burst disk as the valve, you could use a banjo fitting and maybe a support for the barrel to slide on to expedite loading. If you put a nail on the back of your projectile and put in a slightly thicker burst disk than usually needed in a standard burst disk gun. If you projectile was heavy enough, and it sounds like it will be, it will puncture the disk and provide a very efficient valve.
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Unread postAuthor: Hubb » Thu Aug 28, 2008 2:59 pm

BC - no, the barrel pipe is 2". It has a larger inside diameter than Sch40, meaning the intended projectiles will fit looser. And thanks for the warning on the seals and stuff. I'll run tests and, if necessary, I go to regulated CO2.

kablooie - The purpose of this is so I can just drop a round down the barrel. I don't want to have to change the valve after each shot.
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Unread postAuthor: Carlman » Thu Aug 28, 2008 3:01 pm

i did just the same except in marble caliber. drop marble down hits a spring wich hits a nail that has been pushed through which then fires the solnoid valve. (thrs a thread somewhere..)

was pretty cool but broke it down before i discovered propane as a pnue propelent.
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Unread postAuthor: Hubb » Thu Aug 28, 2008 3:06 pm

Edited - My computer went crazy and accidentally double posted.
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Unread postAuthor: trollhameran » Thu Aug 28, 2008 3:11 pm

Its a good idea, would be cool to see it working, just mind your face when you drop the shell into the barrel :wink:
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Unread postAuthor: Hubb » Thu Aug 28, 2008 3:13 pm

Right now, I get a round off about every 15 to 20 seconds. They just don't have the velocity I'm looking for (about 130fps right now).

What I may have to do is just build a piston valve and make the solenoid sprinkler the pilot valve...
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Unread postAuthor: kablooie » Thu Aug 28, 2008 3:37 pm

A piston valve would be cool, and it would provide a lot of power. They do require a little more fiddling though (but you know that...)

If it takes between fifteen and 20 seconds to fill the gun, that's easily enough time to load a burst disk, but it's your call.
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Unread postAuthor: BC Pneumatics » Thu Aug 28, 2008 3:39 pm

Sorry, Hubb, I read :
I use a 2" SDR pipe as the barrel. This allows my ammo made for 2" Sch40 pipe to slide down without obstruction.
incorrectly as:
I use a 2" SDR pipe as the barrel. This allows my ammo made from 2" Sch40 pipe to slide down without obstruction.


Best of luck :)
I myself have been looking into mechanical triggering of the valve lately for 'drop mortar' designs. Safe operation is a large concern if you are not the only one that will be using it.

P.S.: A 555 could help out with that dwell problem. Or a latch that catches the coat hanger cross bar, to be manually released later.

P.P.S.: A larger ratio will likely help with velocity. You may try 1:1, even if it hurts your ROF. Still, the fill time is pretty high, what are you filling from, and is it throttled to control the cooling? (Don't go to fast, PVC is not nice cold) I think I tested with a 2"x24" chamber, and filled in 4-6 seconds, including the valve seating.
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Unread postAuthor: Hubb » Thu Aug 28, 2008 3:59 pm

I considered a 555 timer. That's probably my best route. I'm not to keen on a piston valve assembly for this design.

I'll rebuild it and install a 555 timer, however, I've never used one. Will it allow the current to remain even after the switch is turned off? If it remained for, say, a half second, that should be enough time for the entire chamber to dump.
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Unread postAuthor: BC Pneumatics » Thu Aug 28, 2008 4:17 pm

<a href="http://ourworld.compuserve.com/homepages/Bill_Bowden/555.htm">!Viva Google!</a>
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Unread postAuthor: Sticky_Tape » Thu Aug 28, 2008 4:25 pm

Why not just make it a regular cannon with a ball valve fill and a bipod?
Screw realism just my thought.
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You can tell how awesome a cannon is by the pressure used.
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Unread postAuthor: BC Pneumatics » Thu Aug 28, 2008 4:29 pm

For the same reason we don't simply throw potatoes.
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Unread postAuthor: kablooie » Thu Aug 28, 2008 4:47 pm

Cause we throw like girls?
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